1. Field of the Invention
The present invention relates to a power supply system, and particularly relates to a power supply system using a photocoupler for signal transmission.
2. Description of the Related Art
In a switching power supply configured such that a power input side and a power output side are electrically insulated from each other, an electrically-insulated signal transmission unit such as a photocoupler is used when an output voltage value is fed back to the power input side.
Japanese Patent No. 4682438 describes a switching power supply device. In order to stabilize output voltage, a voltage detection signal of an output voltage detection circuit is transmitted to a control circuit by a photocoupler, and is fed back to control of the pulse width of a drive signal of a MOSFET serving as a switching element in the control circuit. Moreover, when excess output voltage is caused, an overvoltage detection signal of an overvoltage detection circuit is transmitted to the control circuit by another photocoupler to stop operation of the MOSFET.
In some cases other than overvoltage detection as described in Japanese Patent No. 4682438, ON/OFF of a switching power supply may be performed. In order to transmit a command signal for ON/OFF of the switching power supply to a control circuit on a power input side, an electrically-insulated signal transmission unit such as a photocoupler is used, as was expected. In this case, two photocouplers are required, which include a photocoupler for output voltage value feedback and a photocoupler for signal transmission for ON/OFF of the switching power supply.